Hi I would like to ask if I allow dofollow comments in my medical blog so what will be the disadvantage of it? Will it be any problem in my ranking and PR?
I doubt it will affect your PageRank or rankings (unless you will have links to many sites with illegal content), the only disadvantage would be that the link juice will be shared among the all the sites you are linking to so it will be harder to promote your next site (if any).
I would like to suggest allow do follow comments of few days so that people will know your blog and you will get traffic after some times you can use no follow attributes.
The disadvantage is that noobs will flock to your blog to spam it for comment links. Especially if you don't moderate.
Do follow blogs generally attract lots more comments because people are looking to leave their links. So, you might have to waste a bit more time moderating those comments. There is no harm switching, I guess, because you can always switch back if it attracts too many spammers.
This. Plus the fact that most of your comment will be spam and will discourage genuine readers from returning because it makes your site look like an affiliate site and not genuine.
I wouldn't make your blog DoFollow; you'll get tons of spam and it will negatively hurt your search engine ranking (yes, I have personal experience with both). But yes, definitely allow comments and just delete the spam ones you get.